Only four drivers actually use the overlay cores: glint, sunffb, chips, and mga.
They're based on cfb, and as a result they don't support Render and have known
correctness and stability bugs.
The solution is to add additional visuals when Composite is active, and force
windows created on those visuals to always be composited even when no compmgr is
active. This would provide a sort of overlay emulation for every driver using
fb, and would allow the removal of the overlay cores.